This invention relates to blends of Fischer-Tropsch derived fuels and conventional petroleum fuels. More particularly, this invention relates to a blended fuel, useful in a diesel engine which is low in sulfur and demonstrates better than predicted emissions characteristics.
A concern for future diesel fuels is the ability to produce higher quality and cleaner burning diesel fuels without extensive and expensive reprocessing. Typical factors detrimental to fuel quality are high sulfur, high density, high end boiling and T95 points, (the temperature at which most all the material has boiled off, leaving only 5% remaining in the distillation pot) high aromatic and polyaromatic contents. These factors have been shown to have a detrimental effect on emissions. For example, see the Coordinating Research Council (CRC) study on heavy duty diesels in the United States reported in SAE papers 932735, 950250 and 950251, and the European Programme on Emissions, Fuels and Engine Technologies (EPEFE) study on light and heavy duty diesels reported in SAE papers 961069, 961074 and 961075.
In contrast, emissions measurements on Fischer-Tropsch diesel fuels, which have virtually nil sulfur, aromatic and polyaromatic contents demonstrate favorable emissions characteristics. A report by the Southwest Research Institute (SwRI) entitled xe2x80x9cThe Standing of Fischer-Tropsch Diesel in an Assay of Fuel Performance and Emissionsxe2x80x9d by Jimell Erwin and Thomas W. Ryan, III, NREL (National Renewable Energy Laboratory) Subcontract YZ-2-113215, Oct. 1993, details the advantage of Fischer-Tropsch fuels for lowering emissions when used neat, that is, use of pure Fischer-Tropsch diesel fuels.
Presently, there remains a need to develop an economic, low sulfur distillate fuel blend useful as a diesel fuel which has lowered emissions after combustion and allows a greater portion of the distillate to be used as a high value premium product. In particular, sulfur levels, emissions of solid particulate matter (PM), and nitrogen oxides (NOx) are important due to current and proposed environmental regulations. While it has been disclosed that Fischer-Tropsch fuels can be blended with conventional fuels, see for example U.S. Pat. No. 5,689,031 herein incorporated by reference, the ability to further improve such blends with respect to emissions provides a distinct economic advantage.
The citations of the several SAE papers referenced herein are:
P. J. Zemroch, P. Schimmering, G. Sado, C. T. Gray and Hans-Martin Burghardt, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Statistical Design and Analysis Techniquesxe2x80x9d, SAE paper 961069.
M. Signer, P. Heinze, R. Mercogliano and J. J. Stein, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Heavy Duty Diesel Studyxe2x80x9d, SAE paper 961074.
D. J. Rickeard, R. Bonetto and M. Signer, xe2x80x9c, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Comparison of Light and Heavy Duty Dieselsxe2x80x9d, SAE paper 961075.
K. B. Spreen, T. L. Ullman and R. L. Mason, xe2x80x9cEffects of Cetane Number, Aromatics and Oxygenates on Emissions from a 1994 Heavy-Duty Diesel Engine with Exhaust Catalystxe2x80x9d, SAE paper 950250.
K. B. Spreen, T. L. Ullman and R. L. Mason, xe2x80x9cEffects of Cetane Number on Emissions from a Prototype 1998 heavy Duty Diesel Enginexe2x80x9d, SAE paper 950251.
Thomas Ryan III and Jimell Erwin, xe2x80x9cDiesel Fuel Composition Effect on Ignition and Emissionsxe2x80x9d, SAE paper 932735.
M. Hublin, P. G. Gadd, D. E. Hall, K. P. Schindler, xe2x80x9cEuropean Programme on Emissions, Fuels and Engine Technologies-Light Duty Diesel Studyxe2x80x9d, SAE paper 961073.
According to an embodiment of this invention is provided a blended fuel, useful as a diesel fuel, wherein the fuel blend contains an undercut conventional diesel fuel, blended with a Fischer-Tropsch derived diesel fuel, such that the blend demonstrates better than expected emissions and a reduced sulfur content. In particular, the blend is an asymmetric diesel fuel blend comprising a Fischer-Tropsch derived hydrocarbon distillate having a T95 of at least 600xc2x0 F. (316xc2x0 C.), preferably at least 650xc2x0 F. (343xc2x0 C.), more preferably at least 700-750xc2x0 F. (371xc2x0 C.-399xc2x0 C.), blended with a petroleum derived hydrocarbon distillate having an initial boiling point and a T95 no greater than 640xc2x0 F. (378xc2x0 C.), preferably a T95 no greater than 600xc2x0 F. (316xc2x0 C.) wherein the blend has a sulfur content of less than 500 wppm. The resultant diesel fuel blend is characterized by an initial boiling point ranging from at least 280xc2x0 F.+ (138xc2x0 C.+), preferably at least 300xc2x0 F.+ (149xc2x0 C.+), more preferably 320xc2x0 F.+ (160xc2x0 C.+) and a T95 up to about 700xc2x0 F. (371xc2x0 C.), preferably up to about 680xc2x0 F. (360xc2x0 C.), even more preferably up to about 660xc2x0 F. (349xc2x0 C.), still more preferably up to about 640xc2x0 F. (378xc2x0 C.) and contains:
Sulfur  less than 500 wppm, preferably  less than 150 wppm, more preferably  less than 50 wppm, even more preferably  less than 30 wppm,
Polyaromatics  less than 11 wt %, preferably  less than wt 5%, more preferably  less than 1 wt %,
Cetane number  greater than 50, preferably  greater than 55, more preferably  greater than 60,
Density from about 0.79 to about 0.85 wherein the Fischer-Tropsch distillate comprises 5-90 vol. % of the blended diesel fuel, preferably 20-80 vol. %, more preferably 30-80 vol. %.
A typical diesel fuel boils in the range of about 320-700xc2x0 F. However, sulfur levels generally increase with boiling point, i.e., heavier diesel derived from crude oil has a higher sulfur content than lighter diesel. See Jimell Erwin, Thomas W. Ryan, III, xe2x80x9cThe Standing of Fischer-Tropsch Diesel in an Assay of Fuel Performance and Emissionsxe2x80x9d, NREL (National Renewable Energy Laboratory) Subcontract YZ-2-113215, October 1993. The blend of the invention provides a fuel having reduced sulfur levels and emissions levels lower than those predicted by standard correlations, e.g., European Program on Emissions Fuels and Engine Technologies, SAE Paper 961073, by eliminating the heavy end of the conventional diesel fuel and replacing the heavy end with a low sulfur Fischer-Tropsch derived diesel fuel boiling above the range of a normal diesel fuel. In addition to reducing sulfur levels, the diesel fuel blend of this invention outperforms predicted emissions levels, especially in emissions of nitrous oxides.
FIG. 1 is flow-scheme diagram of fixed bed reactors connected in series and contained within an isothermal sand bath for production of a blend stock for use in one embodiment of the present invention.
The Fischer-Tropsch process is well known to those skilled in the art, see for example, U.S. Pat. Nos. 5,348,982 and 5,545,674 herein incorporated by reference. Typically the Fischer-Tropsch process involves the reaction of a synthesis gas feed comprising hydrogen and carbon monoxide fed into a hydrocarbon synthesis reactor in the presence of a Fischer-Tropsch catalyst, generally a supported or unsupported Group VIII, non-noble metal e.g., Fe, Ni, Ru, Co and with or without a promoter e.g., ruthenium, rhenium and zirconium. These processes include fixed bed, fluid bed and slurry hydrocarbon synthesis. A preferred Fischer-Tropsch process is one that utilizes a non-shifting catalyst, such as cobalt or ruthenium or mixtures thereof, preferably cobalt, and preferably a promoted cobalt, the promoter being zirconium or rhenium, preferably rhenium. Such catalysts are well known and a preferred catalyst is described in U.S. Pat. No. 4,568,663 as well as European Patent 0 266 898. The synthesis gas feed used in the process comprises a mixture of H2 and CO wherein H2:CO are present in a ratio of at least about 1.7, preferably at least about 1.75, more preferably 1.75 to 2.5.
Regardless of the catalyst or conditions employed however, the high proportion of normal paraffins in the product produced by the Fischer-Tropsch process must be converted from waxy hydrocarbon feeds into more useable products, such as transportation fuels. Thus, conversion is accomplished primarily by hydrogen treatments involving hydrotreating, hydroisomerization, and hydrocracking in which a suitable fraction of the product is contacted with a suitable catalyst in the presence of hydrogen to isomerize the fraction by converting the molecular structure of at least a portion of the hydrocarbon material from normal paraffins to branched iso-paraffins to form the desired product, as is known to those skilled in the art.
Hydroisomerization and hydrocracking are well known processes for upgrading hydrocarbon synthesis products and their conditions can vary widely. Hydroisomerization is achieved by reacting the waxy feed with hydrogen in the presence of a suitable hydoisomerization catalyst. While many catalysts may be satisfactory for this step, some catalysts perform better than others and are preferred. For example, applicants preferred hydroisomerization catalyst comprises one or more Group VIII noble or non-noble metal components, and depending on the reaction conditions, one or more non-noble metals such as Co, Ni and Fe, which may or may not also include a Group VIB metal (e.g., Mo, W) oxide promoters, supported on an acidic metal oxide support to give the catalyst both a hydrogenation and dehydrogenation function for activating the hydrocarbons and an acid function for isomerization. However, noble metals reduce hydrogenolysis, particularly at lower temperatures and will therefore be preferred for some applications. Preferred noble metals are Pt and Pd. The catalyst may also contain a Group IB metal, such as copper, as a hydrogenolysis suppressant. The cracking and hydrogenating activity of the catalyst is determined by its specific composition. The metal Groups referred to herein are those found in the Sargent-Welch Periodic Table of the Elements, copyright 1968.
The acidic support is preferably an amorphous silica-alumina where the silica is present in amounts of less than about 30 wt %, preferably 5-30 wt %, more preferably 10-20 wt %. Additionally, the silica-alumina support may contain amounts of a binder for maintaining catalyst integrity during high temperature, high pressure processes. Typical binders include silica, alumina, Group IVA metal oxides, e.g., zirconia, titania, various types of clays, magnesia, etc., and mixtures of the foregoing, preferably alumina, silica, or zirconia, most preferably alumina. Binders, when present in the catalyst composition, make up about 5-50% by weight of the support, preferably 5-35% by weight, more preferably 20-30% by weight.
Characteristics of the support preferably include surface areas of 200-500 m2/gm (BET method), preferably about 250-400 m2/gm; and pore volume of less than 1 ml/gm as determined by water adsorption, preferably in the range of about 0.35 to 0.8 m/gm, e.g., 0. 57 ml/gm.
The metals may be incorporated onto the support by any suitable method, and the incipient wetness technique is preferred. Suitable metal solutions may be used, such as nickel nitrate, copper nitrate or other aqueous soluble salts. Preferably, the metals are co-impregnated onto the support allowing for intimate contact between the Group VIII metal and the Group IB metal, for example, the formation of bimetallic clusters. The impregnated support is then dried, e.g., over night at about 100xc2x0-150xc2x0 C., followed by calcination in air at temperatures ranging from about 200xc2x0-550xc2x0 C., preferably 350xc2x0-550xc2x0 C., so that there is no excessive loss of surface area or pore volume.
Group VIII metal concentrations of less than about 15 wt % based on total weight of catalyst, preferably about 1-12 wt %, more preferably about 1-10 wt % can be employed. The Group IB metal is usually present in lesser amounts and may range from about a 1:2 to about a 1:20 ratio respecting the Group VIII metal.
Nevertheless, the Fischer-Tropsch derived distillates that may be used in the blends of this invention include distillates recovered from the Fischer-Tropsch reactor, whether or not hydrotreated, i.e., hydrogen treatments in the presence of a suitable catalyst, including but not limited to, one or more of hydrotreating, hydroisomerization, dewaxing and hydrocracking, as well as distillates recovered from fractionating the wax containing product from the Fischer-Tropsch reactor, whether or not hydrotreated. However, a preferred Fischer-Tropsch derived distillate comprises a distillate fraction derived from any hydroisomerized wax containing Fischer-Tropsch feed utilizing any suitable hydroisomerization catalyst under standard hydroisomerization conditions commonly known in the art.
Preferably, the Fischer-Tropsch derived hydrocarbon distillate has a T95 of at least 600xc2x0 F., more preferably the Fischer-Tropsch derived distillate has an initial boiling point of at least 300xc2x0 F. and a T95 of at least 650xc2x0 F., even more preferably an initial boiling point of at least 320xc2x0 F. and a T95 of at least 700-750xc2x0 F. and contains:
Sulfur, nitrogen  less than 10 wppm, preferably  less than 5 wppm, more preferably  less than 1 wppm,
Aromatics  less than 1 wt %, preferably  less than 0.1 wt %, more preferably undetectable by ASTM D-5292
Cetane number xe2x89xa765, preferably xe2x89xa770,
The conventional petroleum derived fuel may be any conventional low sulfur diesel fuel, i.e., low sulfur No. 2-D diesel fuel as specified in ASTM D-975-98b, which may be derived from crude oil by conventional petroleum processing or from slack wax or from other feed stocks, and is characterized as an undercut diesel fuel, that is, a fuel that has a final cut point below the boiling range of a typical diesel fuel. Preferably, the undercut conventional blend stock of this invention has a T95 no greater than 640xc2x0 F., preferably a T95 below 600xc2x0 F. However, because sulfur level increases with boiling point, cut points may be varied, i.e., decreased, to achieve desired sulfur levels in the conventional blend stock. In this way, sulfur levels of the final blend may be controlled based on the final cut point of the conventional diesel blend stock.